linux-hardening - Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL

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Date:   Fri, 11 Feb 2022 11:58:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Robert Święcki <robert@...ecki.net>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

On February 11, 2022 4:54:26 AM PST, "Robert Święcki" <robert@...ecki.net> wrote:
>> It's mainly about the exit stuff having never been run before on these
>> kinds of process states, so things don't make sense. For example, on the
>> SIGSYS death, the registers have been rewound for the coredump, so when
>> the exit trace runs on x86 it sees the syscall return value as equal to
>> the syscall number (since %rax is used for the syscall number on entry
>> and for the syscall result on exit). So when a tracer watches a seccomp
>> fatal SIGSYS, it sees the syscall exit before it sees the child exit
>> (and therefore the signal). For example, x86_64 write (syscall number
>> 1), will return as if it had written 1 byte. :P
>>
>> So, it's not harmful, but it's confusing and weird. :)
>>
>> > I am trying to figure out if there is a case to be made that it was a
>> > bug that these events were missing.
>>
>> I don't think so -- the syscall did not finish, so there isn't a valid
>> return code. The process exited before it completed.
>
>A tangential point: please ignore for the purpose of fixing the
>problem at hand. I'm mostly making it, in case it can be taken into
>account in case some bigger changes to this code path are to be made -
>given that it touches the problem of signal delivery.
>
>When I noticed this problem, I was looking for a way to figure out
>what syscall caused SIGSYS (via SECCOMP_RET_KILL_*), and there's no
>easy way to do that programmatically from the perspective of a parent
>process. There are three ways of doing this that come to mind.

I had hoped that the parent could read the SIGSYS siginfo_t from the
child, but I haven't found any way to do this. :( :(

I don't seem to be able to use:

- PTRACE_ATTACH to use PTRACE_PEEKSIGINFO on a dead process.
- signalfd (nothing is in the fd after the exit).

Hmpf.

-- 
Kees Cook